Dialed multifrequency signals normally consist of two simultaneous tones, one of a group of high frequency tones, and one of a group of low frequency tones. The individual tones are required to be decoded in twos, whereby the required dialed digit may be distinguished. Decoding has in the past required distinguishing the tone frequency components by filtering. In the event the received signals did not fall within the filter bandwidths, they were not decoded.
To provide accurate decoding, the filters used to distinguish the various frequency tones must be of narrow bandwidth in order that extraneous noise signals would not be translated as tone signals. Further, the filters must be accurately tuned. Due to the tuning, steep filter skirt and bandwith requirements, prior art tone distinguishing filters have been relatively expensive, particularly after including the cost of tuning them individually prior to shipment or being placed in service.